Sensing head and collimator for gamma-camera

ABSTRACT

A detection head and collimator for a gamma camera. The detection head includes several elementary detectors with semiconductors adjacent to each other to form a detection plane. The collimator is placed in front of the detection plane and includes a number of ducts laid out in a repetition pattern. The shape of the elementary detectors and the repetition pattern are rectangular in the detection plane.

BACKGROUND OF THE INVENTION

This invention relates to a detection head and a collimator for a gammacamera and more particularly for a “pixels” gamma camera.

A pixels gamma camera means a camera sensitive to gamma radiation, inwhich the detection head comprises a number of adjacent individualelementary detectors.

The invention has applications in medical imagery, as for example suchas scintigraphy and Single Photo-Emission Computed Tomography (SPECT).

DISCUSSION OF THE BACKGROUND

Gamma cameras conventionally used in medical imagery are of the Angertype. Document (1), listed in the references at the end of thisdescription, contains further information about this subject.

Gamma cameras are used particularly to display the distribution ofmolecules marked by a radioactive isotope previously injected into thepatient, throughout the body or in an organ.

FIG. 1 more precisely shows a detection head 10 of an Anger type gammacamera placed facing an organ 12.

The detection head 10 comprises a collimator 20, a scintillator crystal24, a light guide 22 and several photo-multiplier tubes 26 placedadjacent to each other in order to cover one surface of the light guide22 opposite the scintillator crystal 24. For example, the scintillatormay be an NaI(Tl) crystal.

The collimator is in the form of a lead disk through which a number ofducts 21 carrying gamma radiation can pass, approximately identical andparallel to each other. The disk is placed in contact with thescintillator 24 such that the ducts 21 are perpendicular to the surfaceof this crystal. A divergent or convergent collimator may be used forsome applications in which the object size has to be magnified orreduced to produce the image size.

The function of the collimator 20 is to select the part of the gammaradiation 30 emitted by organ 12 that reaches the detection head atapproximately normal incidence.

The selective nature of the collimator is such that the resolution andsharpness of the image produced can be increased. However, theresolution is increased at the detriment of sensitivity.

The opening and the length of the ducts 21 are determined as a functionof the inspection energy and the compromise between the spatialresolution and the derived sensitivity. As the ducts become longer andnarrower, the spatial resolution of the detection head improves but itssensitivity reduces. Furthermore, the spacing between ducts is chosen tobe higher when the energy of the received radiation is greater.

Known collimator ducts have a hexagonal cross-section (or round for highenergies).

This form is dictated partly by detection uniformity requirements, butalso by collimator manufacturing constraints.

It is considered that the circular shape for the collimator ductcross-section gives the most uniform and homogeneous detection possible.

However, when ducts with a circular cross-section are placed adjacent toeach other, it is observed that the thicknesses of the material wallsseparating the ducts are not uniform. The non-uniform nature of the wallthicknesses and especially the existence of intermediate regions betweenthe ducts in which the thickness of the absorbing material (lead)varies, is a major disadvantage.

Doses of radioactive product injected into the patient necessarily haveto be limited. Thus, the intensity of the emitted radiation isrelatively low. Under these conditions, the extent and thickness ofintermediate walls separating the collimator ducts must be reduced inorder to limit excessive losses of the “useful” radiation.

In order to limit the thickness of the walls between ducts anddifferences in this thickness, collimators are made with ducts with ahexagonal cross-section. This shape also has the advantage that itfacilitates manufacturing of the collimators.

Finally, it can be noted that the hexagonal shape is used to the extentthat it is relatively close to the circular shape, and enablesapproximately uniform detection.

In the case of ducts with a hexagonal cross-section, the thickness ofthe walls that delimit the ducts is usually chosen within a range from0.2 to 2 mm. The characteristic size of the duct opening, in other wordsthe distance between flats in the hexagonal cross-section, is of theorder of 1.5 to 4.5 mm. Finally, the depth of the ducts is usuallychosen between 30 and 50 mm.

Known collimators are usually made using a technique for the assembly oflead sheets shaped to make the ducts. According to another knowntechnique, the collimators may also be obtained by casting in a pinmold.

With reference to FIG. 1, it can be seen that gamma photons that havepassed through the collimator reach the scintillator crystal 24 in whichpractically every gamma photon is converted into several light photons31. Throughout the rest of this text, each detected interaction betweena gamma photon and the detector material, for example with thescintillator crystal, will be denoted as an “event”.

Photo-multipliers 26 are designed to emit an electric pulse proportionalto the number of light photons received on scintillator 24, at eachevent.

In order to be able to locate the scintillation event more precisely,the photo-multipliers 26 are not placed immediately adjacent toscintillator crystal 24, but are separated from it by the light guide22.

The photo-multipliers emit a signal, the amplitude of which isproportional to the total quantity of light produced in the scintillatorby gamma radiation, in other words proportional to its energy. However,the individual signal from each photo-multiplier also depends on thedistance that separates it from the interaction point 30 of the gammaradiation with the scintillator material. Each photo-multiplier outputsa current pulse proportional to the light flux that it received. In theexample shown in FIG. 1, small graphs A, B, C show that: thephoto-multipliers 26 a, 26 b and 26 c located at different distancesfrom an interaction point 30 output signals with different amplitudes.

The position of the interaction point 30 and the energy of a gammaphoton is calculated in the gamma camera starting from signalsoriginating from all photo-multipliers by making a center of gravityweighting of the contributions of each photo-multiplier.

However, Anger type gamma cameras have a disadvantage due to the factthat the number of light photons created during each event in thescintillator crystal satisfies Poisson statistics. The number ofphoto-electrons torn from the photo-cathode of the photo-multipliersalso satisfies Poisson statistics. Thus, the position and energycalculations are affected by an inaccuracy related to Poissonfluctuations in the number of light photons and the number ofphotoelectrons produced for each event.

The standard deviations of the fluctuations is lower when the number ofphotons or photoelectrons is high. The inherent spatial resolution ofthe gamma camera is characterized by the width at mid-height of thedistribution of the calculated positions, for a single isolatedcollimated point source placed on the crystal. The resolution is of theorder of 3 to 4 mm at 140 keV. Furthermore, the energy of the gammaphoton is calculated by taking the sum of the contributions of allphoto-multipliers that received light. This is also affected by astatistical fluctuation. The energy resolution is characterized by theratio of the mid-height width of the distribution of calculatedenergies, to the average value of the distribution, for the same source.It is of the order of 9 to 11% at 140 keV.

Detection heads for gamma cameras are also known in which thescintillator crystal and photo-multipliers are replaced by soliddetectors arranged in the form of a matrix of individual detectors. Inthis case, the spatial resolution of the gamma camera depends on thesize of the individual detectors.

The attached FIG. 2 very diagrammatically shows a detection head withsolid detectors, for information. The detection head 40 comprisesseveral individual elementary detectors 42 with semi-conductors. Forexample, they may be CdTe or CdZnTe type detectors. Individual detectorsare approximately identical to each other and are placed adjacent toeach other in the form of a matrix network to form a detection plane 44.

Furthermore, detectors 42 are located on a printed circuit board 46 andare connected to preamplifiers (not shown) on this board. Board 46collects detection signals from the various individual detectors, shapesthem and then sends them to a calculation and information processingunit 48. This unit calculates the position and energy of events. Adetection head like that shown in FIG. 2 has a significantly betterenergy resolution than the detection head shown in FIG. 1, since thenumber of charges created in the semiconductor is 10 times greater thanthe number of light photons created in the scintillator crystal.

A gamma camera on which a detection head conform with FIG. 2 is fitted,may also be fitted with a collimator to select radiation approximatelyperpendicular to the detection head.

FIG. 3 shows a partial top view of a gamma camera with solid detectorson which a conventional collimator is fitted.

This figure shows the individual detectors 42 placed adjacent to eachother to form the detection plane 44. Elementary individual detectorshave a central part 50 sensitive to gamma radiation, and a peripheraledge (as small as possible) 52 not sensitive to gamma radiation.

A collimator 20, similar to that in FIG. 1, is located above thedetection plane 44. It comprises several ducts 21 with hexagonalcross-sections, placed adjacent to each other and with a main axisapproximately perpendicular to the detection plane 44. Each duct isdelimited by a lead sheet folded into a hexagonal shape. The lead sheetsin this shape are placed adjacent to each other and fixed together toform the collimator 20. This type of “honeycomb” structure isparticularly easy to make and is well known for the manufacture ofcollimators like those used on “Anger” type cameras. The collimator canalso be cast in a mold containing pins with a hexagonal cross-section.

Although operation of a detection head conform with FIG. 3 is generallysatisfactory, the inventors have observed that its sensitivity is notuniform, and have found that this non-uniformity is due to an unequalcoverage of elementary detectors by collimator ducts.

This problem is demonstrated and illustrated in FIGS. 4A, 4B, 4C and 4D.

FIGS. 4A to 4D show a top view of different relative positions of anindividual detector 42 on the detection head in FIG. 3, with respect tothe ducts 21 in collimator 20. For simplification reasons, a singleindividual detector and only part of the collimator are shown.Furthermore, the scale in FIGS. 4A to 4D is slightly greater than thescale in FIG. 3.

Calculations of the area facing the sensitive part of the detector withcollimator ducts have been carried out using detectors for which theshape of the sensitive part is square with a 4 mm long side l, and forhexagonal ducts with the distance d between flats equal to 1.5 mm.

These calculations show that the facing area is 12.6 mm², 12.2 mm², 11.8mm² and 12.6 mm ², for the configurations in FIGS. 4A to 4Drespectively.

To avoid non-uniformity problems in the response of the detection head,the dimensions of the hexagonal ducts can be adapted so that they coverthe same active surface area of each individual detector. This requiresthe manufacture of a collimator using ducts with an irregular hexagonalshape. However, the manufacture of this type of collimator is difficultand expensive.

SUMMARY OF THE INVENTION

Thus, one purpose of this invention is to propose a detection headwithout the limitations and the disadvantages mentioned above.

Another purpose of the invention is to propose a detection head withindividual detectors with semi-conductors and a uniformly-shapedcollimator with uniform sensitivity.

Another purpose is to propose a collimator for a detection head withadjacent individual detectors enabling uniform detection.

In order to achieve these purposes, the purpose of the invention is moreprecisely a gamma camera detection head comprising:

several elementary detectors with semiconductors, approximatelyidentical and placed adjacent to each other to form a detection plane,and,

a collimator placed in front of the detection plane and consisting of anumber of ducts for carrying gamma radiation, approximately identical toeach other and laid out in a repetition pattern;

and in which the shape of the elementary detectors and the repetitionpattern are rectangular in the detection plane, the length and width ofthe repetition pattern being sub-multiples of the length and width ofthe elementary detectors.

For the purposes of this invention, the length and width of the ductrepetition pattern means the length and width of the ducts, includingthe thickness of the material walls that delimit the ducts. In the sameway, the length and width of the elementary detectors means the lengthand width of their sensitive parts including the thickness of anyinsensitive “dead” areas surrounding the sensitive parts.

If insert walls are placed between the elementary detectors, the lengthand width of the elementary detectors are assumed to include thethickness of these walls.

Finally, note that the term rectangle is understood to be the shape of aquadrilateral in which the four corners are right angles. Thus, theshape referred to as “rectangular” also includes a square shape which isonly a special case of a rectangular shape.

According to the characteristics of the invention, it is found that thesensitive area of the detector facing the collimator ducts isapproximately identical for each elementary detector.

This can give excellent uniformity in the response of the detection headwith this equipment.

According to particular embodiments of the detection head, the shape ofthe elementary detectors and/or the shape of the repetition pattern issquare.

The invention also relates to a collimator for a gamma camera. Thecollimator according to the invention contains several ducts forcarrying gamma radiation, approximately identical and parallel to eachother, the ducts having a square transverse cross-section.

Finally, the invention relates to a gamma camera comprising a collimatoror a detection head like those described above.

Other characteristics and advantages of the invention will becomeclearer from the following description with reference to the figures inthe attached drawings, given for illustrative purposes and in no wayrestrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1, described above, is a simplified diagram showing the operationof an Anger type camera equipped with a collimator.

FIG. 2 diagrammatically shows a detection head with semi-conductors forgamma camera, with several individual detectors.

FIG. 3 is a partial top view of a detector conform with FIG. 2 equippedwith a collimator with ducts with a hexagonal cross-section.

FIGS. 4A to 4D show details of different types of possible coveragesbetween individual detectors in the detection head and collimator ducts.

FIG. 5 is a partial diagrammatic top view of a detection head accordingto the invention.

FIG. 6 is a partial diagrammatic top view of a variant embodiment of adetection head according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following description, parts that are identical to or similar toparts in the figures described above will have the same references plus100.

As stated above, FIG. 5 is a partial top view of a detection head 140according to the invention.

The detection head comprises a number of elementary detectors 142 placedadjacent to each other in the form of a matrix network to form adetection plane 144.

The elementary detectors 142 are CdTe or CdZnTe type detectors withsemi-conductors and have a square area in the detection plane 144 with aside dimension of 3 to 5 mm.

In the example described, the surface of each detector in the detectionplane 144 has a central part 150 that is sensitive to gamma radiationand an insensitive peripheral edge 152. The sensitive central part alsohas a square surface with a side dimension of 3 to 5 mm. This dimensionis shown in the figure with reference L. The thickness E of theperipheral edge 152 is of the order of 3 mm.

The detection head also comprises preamplifier circuits to collectsignals output from detectors 142 and to send them to a processing unit.

These elements are not shown in FIG. 5 for reasons of clarity.

The detection head 140 also comprises a collimator 120 placed in frontof the detection plane 144. The collimator 120 may be placed directly incontact with the detection plane 144.

The collimator has a number of ducts 121 laid out perpendicular to thedetection plane, in order to carry gamma radiation. Note that the ductsare not necessarily perpendicular to the detection plane, but may form adivergent or convergent bundle for a particular application.

Ducts 121 are placed adjacent to each other and are laid out accordingto a repetition pattern of the individual ducts, each of which has asquare cross-section in the detection plane.

The side of each square cross-section is a sub-multiple of the side ofthe square area of the individual detectors.

In the case shown in FIG. 5, the length and width of the repetitionpattern are equal to one third of the length and width of the elementarydetectors in the detection plane. Thus, 9 ducts including their walls,fit into the area of each detector.

In the particular example given, each square duct 121 has an openingwith a side e₁ of the order of 1.33 mm, and is delimited by a wall 123with a thickness e₁ of the order of 0.1 mm.

It can be checked that the proportion of the area of the sensitivecentral part 150 intercepted by the ducts is the same for eachelementary detector 142. Thus good detection uniformity is achieved.

FIG. 6 shows a variant embodiment of the detector according to theinvention.

In the case in FIG. 6, the side (i.e., the width and length of therepetition pattern) is equal to half the side of each elementarydetector in the detection plane.

The elements shown in FIG. 6, except for their dimensions, are similarto those in FIG. 5. They are denoted by the same references, and theabove description contains information about them.

In the example in FIG. 6, each duct has an opening with a side l₂ equalto 1.7 mm and is surrounded by a wall 123 with a thickness e₂ of 0.3 mm.

Advantageously, collimators 120 like those shown in FIGS. 5 and 6 may bemade by electro-erosion from a solid block of absorbing materials suchas a solid block of lead. Electro-erosion pins with a shapecorresponding to the shape of the ducts are pushed forwards into theblock to form the ducts. This method facilitates manufacture of ductswith a square cross-section, and can produce sharp angles.

According to a variant, collimators conform with the invention can alsobe made by molding. In this case, the ducts are defined by pins with asquare cross-section. These pins are preferably slightly pyramid-shapedto facilitate removing collimators from the mold.

What is claimed is:
 1. Gamma camera detection head comprising:elementary semiconductor detectors, approximately identical and placedadjacent to each other to form a detection plane; and a collimatorplaced in front of the detection plane, including a number of ductsconfigured to carry gamma radiation, each duct approximately identicalto each other and laid out in a repetition pattern, wherein a shape ofthe elementary detectors and the repetition pattern are rectangular inthe detection plane and a length and a width of the repetition patternare sub-multiples of a length and a width of the elementary detectors.2. Detection head according to claim 1, wherein at least one of theshape of the elementary detectors and the shape of the repetitionpattern is square in the detection plane.
 3. Detection head according toclaim 2, wherein the width and length of the repetition pattern is equalto one third of the width and length of the individual detectors. 4.Detection head according to claim 2, wherein the width and length of therepetition pattern is equal to half the width and length of theindividual detectors.
 5. Gamma camera collimator comprising: a number ofducts configured to carry gamma radiation to elementary detectors, saidducts approximately identical with and parallel to each other and laidout in a repetition pattern, the ducts having a square transversecross-section and having a length and a width of the repetition patternwhich are sub-multiples of a length and a width of the elementarydetectors.
 6. Gamma camera comprising a collimator according to claim 5.7. Gamma camera comprising a detection head according to one of claims 1to 4.